This invention relates to a process for the separation of a gas containing hydrocarbons.
Ethylene, ethane, propylene, propane, and/or heavier hydrocarbons can be recovered from a variety of gases, such as natural gas, refinery gas, and synthetic gas streams obtained from other hydrocarbon materials such as coal, crude oil, naphtha, oil shale, tar sands, and lignite. Natural gas usually has a major proportion of methane and ethane, i.e., methane and ethane together comprise at least 50 mole percent of the gas. The gas also contains relatively lesser amounts of heavier hydrocarbons such as propane, butanes, pentanes and the like, as well as hydrogen, nitrogen, carbon dioxide and other gases.
The present invention is generally concerned with the recovery of ethylene, ethane, propylene, propane, and heavier hydrocarbons from such gas streams. A typical analysis of a gas stream to be processed in accordance with this invention would be, in approximate mole percent, 85.6% methane, 6.9% ethane and other C.sub.2 components, 3.0% propane and other C.sub.3 components, 0.5% iso-butane, 1.2% normal butane, 1.1% pentanes plus, with the balance made up of nitrogen and carbon dioxide. Sulfur containing gases are also sometimes present.
The historically cyclic fluctuations in the prices of both natural gas and its natural gas liquid (NGL) constituents have at times reduced the incremental value of ethane, ethylene, propane, propylene, and heavier components as liquid products. Competition for processing rights has forced plant operators to maximize the processing capacity and recovery efficiency of their existing gas processing plants, as well as to provide the ability to more quickly respond to changing market conditions. Available processes for separating these materials include those based upon cooling and refrigeration of gas, oil absorption, and refrigerated oil absorption. Additionally, cryogenic processes have become popular because of the availability of economical equipment that produces power while simultaneously expanding and extracting heat from the gas being processed. Depending upon the pressure of the gas source, the richness (ethane, ethylene, and heavier hydrocarbons content) of the gas, and the desired end products, each of these processes or a combination thereof may be employed.
The cryogenic expansion process is now generally preferred for natural gas liquids recovery because it provides maximum simplicity with ease of start up, operating flexibility, good efficiency, safety, and good reliability. U.S. Pat. Nos. 4,157,904, 4,171,964, 4,251,249, 4,278,457, 4,519,824, 4,617,039, 4,687,499, 4,689,063, 4,690,702, 4,854,955, 4,869,740, 4,889,545, 5,275,005, 5,555,748, and 5,568,737, reissue U.S. Pat. No. 33,408, co-pending application Ser. No. 08/696,114, and co-pending application Ser. No. 08/738,321 describe relevant processes (although the description of the present invention in some cases is based on different processing conditions than those described in the cited U.S. Patents).
In a typical cryogenic expansion recovery process, a feed gas stream under pressure is cooled by heat exchange with other streams of the process and/or external sources of refrigeration such as a propane compression-refrigeration system. As the gas is cooled, liquids may be condensed and collected in one or more separators as high-pressure liquids containing some of the desired C.sub.2 + (or C.sub.3 +) components. Depending on the richness of the gas and the amount of liquids formed, the high-pressure liquids may be expanded to a lower pressure and fractionated. The vaporization occurring during expansion of the liquids results in further cooling of the stream. Under some conditions, pre-cooling the high pressure liquids prior to the expansion may be desirable in order to further lower the temperature resulting from the expansion. The expanded stream, comprising a mixture of liquid and vapor, is fractionated in a distillation (demethanizer or deethanizer) column. In the column, the expansion cooled stream(s) is (are) distilled to separate residual methane, nitrogen, and other volatile gases as overhead vapor from the desired C.sub.2 components and heavier hydrocarbon components as bottom liquid product (or to separate residual methane, C.sub.2 components, nitrogen, and other volatile gases as overhead vapor from the desired C.sub.3 components and heavier hydrocarbon components as bottom liquid product).
If the feed gas is not totally condensed (typically it is not), the vapor remaining from the partial condensation can be split into two or more streams. One portion of the vapor is passed through a work expansion machine or engine, or an expansion valve, to a lower pressure at which additional liquids are condensed as a result of further cooling of the stream. The pressure after expansion is essentially the same as the pressure at which the distillation column is operated. The expanded stream is supplied as a feed to the column in a lower region of an absorption section contained in the distillation column and is contacted with cold liquids to absorb the C.sub.2 (or C.sub.3 ) components and heavier components from the vapor portion of the expanded stream.
The remaining portion of the vapor is cooled to substantial condensation by heat exchange with other process streams, e.g., the cold residue gas. Some or all of the high-pressure liquid may be combined with this vapor portion prior to cooling. The resulting cooled stream is then expanded through an appropriate expansion device, such as an expansion valve, to a pressure slightly above that at which the demethanizer (or deethanizer) column is operated. During expansion, a portion of the liquid will vaporize, resulting in cooling of the total stream. The flash expanded stream is then directed in heat exchange relation with the overhead distillation stream from the demethanizer (or deethanizer), cooling the distillation stream and condensing at least a portion of it, whereupon the warmed expanded stream is supplied to the middle or lower region of the absorption section in the distillation column. The condensed liquid in the cooled distillation stream is removed, leaving the volatile residue gas containing substantially all of the methane (or substantially all of the methane and C.sub.2 components). The condensed liquid stream is then supplied to the distillation column as a top column feed so that the cold liquids contained in the stream can contact the vapor portions of the expanded stream and the warmed expanded stream in the absorption section of the distillation column.
The purpose of this process is to perform a separation that produces a residue gas leaving the process which contains substantially all of the methane in the feed gas with essentially none of the C.sub.2 components and heavier hydrocarbon components (or substantially all of the methane and C.sub.2 components in the feed gas with essentially none of the C.sub.3 components and heavier hydrocarbon components), and a bottoms fraction leaving the demethanizer (or deethanizer) which contains substantially all of the C.sub.2 components and heavier hydrocarbon components with essentially no methane or more volatile components (or substantially all of the C.sub.3 components and heavier hydrocarbon components with essentially no methane, C.sub.2 components or more volatile components). The present invention provides a means for modifying an existing processing plant to achieve this separation at substantially lower capital cost by eliminating much of the equipment associated with providing reflux for the absorption section of the demethanizer (or deethanizer) column. The present invention, whether applied in a new facility or as a modification to an existing processing plant, can be quickly and easily adjusted to either recover C.sub.2 components in the bottom liquid product, or to reject C.sub.2 components to the volatile residue gas while recovering nearly all of the C.sub.3 components and heavier hydrocarbons in the bottom liquid product. This processing flexibility allows the plant operator to respond to fluctuations in natural gas and ethane prices by operating the processing plant in the manner that produces the highest product revenues.
In accordance with the present invention, it has been found that C.sub.2 recoveries in excess of 86 percent can be maintained while providing essentially complete rejection of methane to the residue gas stream. In addition, it has been found that C.sub.3 recoveries in excess of 97 percent can be maintained while providing essentially complete rejection of C.sub.2 components to the residue gas stream. The present invention, although applicable at lower pressures and warmer temperatures, is particularly advantageous when processing feed gases at pressures in the range of 600 to 1000 psia or higher under conditions requiring column overhead temperatures of -50.degree. F. or colder.